Design of erodible and non-erodible, alluvial channels-

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Lecture 13 Design of erodible and non-erodible,
alluvial channels- Kennedys and Laceys
theories.
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According to Kennedy the critical velocity ratio
Vc in a channel may be defined as the mean
velocity of flow which will just keep the channel
free from silting or scouring. His investigations
pertain to Upper bari Doab canal in UP.
m Critical velocity ratio 1.1 to 1.2 for
coarse sand 0.8 to 0.9 for fine sand
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KENNEDYS METHOD OF CHANNEL DESIGN PROCEDURE

Q A x V
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• Assume a depth of flow d, m
• Compute the critical velocity from kennadys
  formula
• Compute are of c/s of flow Q/Vc
• Assuming a side slope of channel, say 0.51
  compute the bed width
• Compute the wetted perimeter for the assumed
  depth abd computed bed width
• Calculate C from Kutters formula and then the
  velocity of flow by Chezys equation
• If the Velocity computed now is same as found by
  kennadys method the design depth is correct
• Otherwise repeat the above steps by assuming
  different depth of flow

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CWPC PRACTICE FOR n
Type of soil Canal discharge (cumecs) Value of n
1. Soil other than rock Up to 0.014 0.14 to 1.4 1.4 to 14 Above 14 0.03 0.025 0.0225 0.020
2. Rocky cuts 1. When rock portion at least 15 cm above the excavated bed level is left out in working out cross sectional area. 0.035 to 0.05
2. When no portion above bed level is left out 0.05 to 0.080

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Channel of condition Value of n
1. Very good 0.0225
2. Good 0.025
3. Indifferent 0.0275
4. Poor 0.03
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LACEYS REGIME THEORY The regime theory
postulates that a channel with erodible
boundaries tends to adjust the dimensions viz.,
width, depth and slope in order to attain a state
of equilibrium called REGIME STATE. Two regimes
initial and final Initial regime occurs
immediately after constructing and putting the
channel under use by adjustment of bed
form. Final regime occurs after long time
adjustment of
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Two regimes initial and final Initial regime
occurs immediately after constructing and putting
the channel under use by adjustment of bed form
by silting or scouring. Final regime occurs
after long time adjustment of bed width and
banks. When the channel reaches a regime
condition the channel conveys water smoothly
without deposition or scouring.
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Laceys theory
Silt factor
Where, m mean particle size, mm
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The difference between Kennedys and Laceys
theory is that Kennedy considered the depth of
flow (d) as significant variable and Lacey
considered the Hydraulic radius (R) as the
significant variable
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• Design procedure
• Q and m are initially known
• Calculate the silt factor f
• Compute V from Lazeys equation
• Compute A from continuity equation
• Compute P S from Lazeys equations